The trend in recent years toward smaller and lighter electronic devices has strengthened the demand for high-energy densification of secondary cells. Lithium secondary cells have been receiving attention as high-energy densified secondary cells aimed at meeting this demand, and they have been undergoing rapid development.
The main problem faced in the development of these lithium secondary cells is that the lithium metal used as the negative electrode undergoes deterioration with repeated charging and discharging. This is a result of the state of the lithium metal as it is deposited on the negative electrode during charging; for example the dendrite, which is a dendritic crystal, causes delamination from the negative electrode plate or short-circuiting with the opposing electrode.
In order to overcome these problems, a variety of different negative electrode materials comprising lithium alloys and doped/undoped lithium have been proposed and tested.
However, problems remain with lithium alloys, in that deep charging and discharging is difficult, they are not suitable for charging and discharging with a high current density, etc. On the other hand, carbon materials and electrically conductive polymers have also been proposed as lithium doping materials, but remaining problems with such electrically conductive polymers include that they have low doping ratios, and that they are chemically and electrically unstable with lithium. At present, most attention is being directed to carbon materials as lithium metal substitutes for the negative electrodes of lithium secondary cells.
When a carbon material is used as the negative electrode of a lithium secondary cell, lithium is intercalated between layers of the carbon material in the electrolyte solution during charging, to form what is known as a graphite intercalation compound. Also, during discharge the interlayer lithium is discharged into the electrolyte solution. Thus, in principle, the use of a carbon material makes it possible to eliminate the deterioration of the negative electrode which accompanies the charge/discharge cycle of the dendrite, etc. that occurs when lithium metal is used as the negative electrode.
Such carbon materials for lithium secondary cell negative electrodes that have been studied include thin-film carbonaceous electrodes prepared by CVD techniques (Japanese Unexamined Patent Application No. 63-24555, etc.), coke powder (Japanese Unexamined Patent Application Nos. 1-204361 and 1-221859) and resins and other carbonized polymers (Proc. Prim. Second. Amb. Temp. Lithium Batteries, p.530-539).
Nevertheless, the above-mentioned carbon materials presently have a small electrical capacity per unit weight. The electrical capacity corresponds to the amount of lithium inserted into the carbon material. Although the amount of inserted lithium is theoretically limited to a maximum of one lithium atom per 6 atoms of carbon (C.sub.6 Li, 372 mAh/g), in practice it has been found that the carbon materials described above can exhibit a capacity of no more than about 250 mAh/g (see, for 12 example, Proceedings of the 31st Battery Simposium in Japan, 3B11 (1990); and Proceedings of the 32nd Battery Simposium in Japan, 2B12 (1992)).
There are also examples of carbon fibers being employed as electrodes (Japanese Unexamined Patent Application Nos. 63-268056, 62-268058, 63-10462, 64-14869), but because of their filamentous shape, the formation density cannot be improved and thus it has been impossible to increase their volume density when employed in an electrode.
Here, the present inventors have found, as a result of diligent research aimed at developing a carbon material with a high cycle stability for repeated charging and discharging, that the shape of the carbon material is a particularly important factor, with a filamentous shape being the most suitable, and that by adjusting the degree of graphitization and the shape of a particulate carbon material prepared by pulverizing carbon fibers derived from pitch, a very high effectiveness may be achieved from the viewpoint of increasing the lithium doping ratio and initial charge/discharge efficiency, and the present invention has been completed based upon this finding.
Consequently, it is an object of the present invention to develop a carbon material with a large lithium doping ratio and a high charge/discharge efficiency, as well as a method for its production.
It is another object of the present invention to provide a material for the negative electrode of a lithium secondary cell which has a large discharge capacity and a long cycle life as a result of the above-mentioned excellent lithium doping ratio and charge/discharge efficiency, as well as a lithium secondary cell which employs this negative electrode material.